I don't honestly see how this would be possible to do as a false flag, at least in terms of some FSB or other official types posing as terrorists - the amount of changes to normal operating procedures that would be required just to get the attack to take place would be so many that dozens / hundreds of cops, other first responders, security guards and normal members of the public would notice. If one cop got a lucky shot off and killed / disabled one fake terrorist it would risk everything coming out. The tower block attacks were rumbled because of unusual security force behaviour and they were much less risky (from the point of view of exposure) attacks than this appears to have been.
If Ukraine / Ukrainian interests were genuinely behind this then Western support will vanish. Even the most cretinous of the NAFO crowd, to say nothing of hawkish Washington types, would surely balk at legitimising this sort of thing being a means of warfare between states.